A typical lightwave communications system consists of an optical transmitter, which is usually a semiconductor laser diode (emitting in the invisible infrared region of the optical spectrum) with associated electronics for modulating it with the information bearing signals; a transmission channel—namely the optical fiber which carries the modulated light beam; and finally, a receiver, which consists of an optical detector and associated electronics for retrieving the signal.
The optical transmitter in any lightwave communications system performs the functions of generating an optical wave and impressing information on the wave by modulation. A laser light source is commonly used in high speed optical fibre communication systems. Chromatic dispersion of the optical fibre transmission medium requires that the optical spectrum emitted by the laser light source be narrow to avoid distortion of the propagating optical waveform. Modulating the output intensity of a semiconductor laser diode using excitation current typically results in spectral broadening (chirping) of the transmitted optical signal. This chirping limits the tolerance of the optical signal to fibre dispersion and, therefore, limits the transmission distance and/or bit-rate of the system.
Chirp and turn-on transient effects associated with direct modulation of a semiconductor laser diode can be avoided completely if the light generation and modulation processes are separated. The laser can be allowed to operate continuous wave (CW), and an external modulator is interposed between the laser and the node output. Chirping of the external modulator can be controlled to optimize the optical signal for the specific system design.
A common way of implementing intensity or amplitude modulation is to use waveguides as the arms of a Mach-Zehnder interferometer with electrodes deposited alongside or on top of the arms as shown in FIG. 1. The waveguides 10 are connected at both the input and output ends by Y-branches 11, 12. Input light waves to the input Y-branch 11 divide the power equally among the two waveguides 10. They recombine at the output Y-branch 12. With no voltage applied to the electrodes, the input and output light-waves are in phase to present an intense output. By applying enough voltage to change the phase difference between the branches to π radians, the output light intensity becomes zero. Any voltage in between will change the intensity accordingly, thus achieving intensity modulation. More generally, the transmission function of the interferometer is proportional to the square cosine function of the differential phase shift in the two arms.
The most useful external modulators are based on a voltage-dependent phase retardation in some material, either semiconductor, insulating crystal, or organic polymer. With crystals or anisotropic polymers, the electro-optic effects is used, i.e. the voltage dependence of refractive index. That is, many crystals will respond to an applied electric field in certain crystal orientations to produce changes in the refractive index of the crystal. The modulator structure is usually implemented as a waveguide lithographed in or on a substrate material.
To date, most external modulators have been implemented as separate devices in lithium niobate (LiNbO3), which has a very high electro-optic coefficient along certain axes. Lithium niobate has a large temperature coefficient of expansion which can result in an effective imbalance in optical path length between the two arms of a Mach-Zehnder interferometer. Specifically, the bias characteristics of a LiNbO3 Mach-Zehnder interferometer change with temperature and aging of the device. Optical transmitters using this technology for external modulation are required to search for and control to the appropriate bias level. This search results in optical power transients during transmitter card initialization.
A single wavelength laser diode may be operated at a low bias current to reduce the optical output power during the search for external modulator bias conditions. However, this technique has two limitations.
Firstly, the output wavelength of the laser diode is typically dependent on the laser bias current. In a dense wavelength division multiplexed (DWDM) system it is critical that the transmitter emit light only at the wavelength of the desired channel. It is, therefore, necessary to compensate for the change in wavelength due to bias current with a change in laser operating temperature (temperature tuning of the laser wavelength). This technique cannot be applied if the CW light source is a tunable laser diode which uses the laser operating temperature as a wavelength tuning mechanism. Secondly, it may not be possible to reduce the laser output power sufficiently to eliminate the effects of optical power transients on the system.
The single wavelength laser diodes used in most transmitter realizations to date are usually of sufficient power such that the use of optical amplifiers to boost the optical signal is not warranted. The wavelength and output power of such transmitters are not readily controllable as mentioned above. However, the need for precise control of transmitter output wavelength and optical power is critical in current DWDM systems where channels are packed closer and closer together to meet the ever-increasing demands for more and more capacity.